Detector arrangement and corresponding operating method

ABSTRACT

The invention concerns a detector arrangement for detection of radiation, in particular particle radiation or electromagnetic radiation, with a semi-conductor detector with several pixels for detection of the radiation. It is proposed that the individual pixels each have a first subpixel ( 1 ) and a second subpixel ( 2 ). The semi-conductor detector can be switched between a first collection state, in which the first subpixel ( 1 ) is sensitive and the second subpixel ( 2 ) is insensitive so that radiation-generated signal charge carriers are substantially collected only in the first subpixel ( 1 ), and a second collection state in which the second subpixel ( 2 ) is sensitive and the first subpixel ( 1 ) is insensitive so that the radiation-generated signal charge carriers are collected substantially only in the second subpixel ( 2 ). The invention furthermore concerns a corresponding operating method and detector arrangements based on the same concept with a higher number of subpixels per pixel.

FIELD OF THE INVENTION

The invention concerns a detector arrangement for detection ofradiation, in particular particle radiation or electromagneticradiation. Furthermore the invention concerns a corresponding operatingmethod for such a detector arrangement.

BACKGROUND OF THE INVENTION

Whenever imaging sensors are used for quantitative analysis of incidentradiation, precise control of the exposure time, i.e. the time duringwhich the sensor is exposed to the incident radiation, is of essentialimportance. Either the exposure of the sensor must be as even aspossible over its surface, or the inhomogeneities caused by the shuttermust be known so precisely that subsequent calibration can be carriedout.

In many sectors in which high speeds are not required, mechanicalshutters are used. These however do not have arbitrarily short shuttertimes or arbitrarily high repeat rates, and their life is limitedbecause of the mechanical wear.

Electro-optical shutter elements, such as for example acoustic-opticalmodulators, indeed have significantly faster time behavior but asattachments with separate control, they are not compact and thereforenot suitable for all applications. The same applies to controllableimage amplifier tubes.

New developments in the sector of imaging sensors however allowintegration of the shutter into the sensor itself in the form of an“electronic” shutter: the radiation is no longer physically isolatedfrom the sensor surface, but the signal is either recorded or rejectedat the sensor itself.

Such an electronic shutter circuit cannot however be used for every typeof imaging sensor. For CCD-based (CCD=Charge-Coupled Device) sensors forexample, such a mechanism cannot be used. Local intermediate stores helpbut these only have finite storage capacity and also impose a loss ofsensitivity.

Concepts on the basis of so-called active pixel sensors (APS) howeverare widely used. Depending on implementation, such an electronic shuttercan be switched with very high cycle rates and very good homogeneityover the sensitive area of the corresponding sensor.

However, all systems currently used for implementing an electronicshutter, using the mechanical or electro-optical concepts specifiedabove, share the common feature that the sensor is completelyinsensitive outside the opening time of the shutter. The shuttertherefore leads to a sensor dead time. This is always a problem fortime-continuous measurement, i.e. if images must be recorded in directsuccession without dead time and with defined exposure time.

A shutter mechanism may however assume a further function beyond controlof the exposure time. It can be used to prevent any signals which arereceived during the read process from disrupting the signal evaluationand falsifying the signals.

The signal detected by the actual sensor during the exposure time mustbe extracted from the sensor for processing, then amplified anddigitized. Depending on the required precision, correspondingly longerprocessing times are required. If new signals are received duringprocessing of the signals already present, the signal amplitude isfalsified. The nature and extent of the falsification to be expectedgreatly depend on the sensor used.

For CCDs for example, the working cycle is divided into exposure timeand transfer time. During the exposure time, charges generated byincident photons are integrated. During the transfer time, the signalcharge is transferred to the read amplifier and amplified there. Fortime-critical applications, the CCD is read in parallel columns, i.e.each CCD column has its own amplifier channel, whereby the read-outspeed can be multiplied approximately by the number of columns. Despitethis, additional signals reaching the sensor during the charge transferare not assigned correctly either in time or in position. These areso-called out-of-time events (OOT).

The abovementioned long signal processing times mean that the total readtime is determined not only by the pure charge transfer, but quitesignificantly by the signal processing. Therefore attempts are made todecouple the transfer time from the signal processing time by creatingan intermediate store (frame store) by doubling the number of pixels.There is now a sensor region and a frame store region. The signal chargeis (quickly) transferred from the sensor region to the frame storeregion, and from there can be shifted (slowly) to the amplifiersarranged at the matrix end and amplified while another charge isintegrated in the sensor region.

This complex measure indeed significantly reduces the number of OOTs butcannot suppress these completely because of the finite transfer timebetween sensor and frame store region. However in this case too, theintegration time cannot be selected freely since the integration timemust be at least as long as the read-out time.

Implementation of an electronic shutter in the CCD is not possible fordesign reasons. An external shutter can completely suppress any OOTsoccurring, but again leads to a dead time.

As already stated, in a CCD the necessary transfer of the signalcharge—and indeed of the entire collected charge in the sensor region—tothe read node can, in the least favorable case, take place over theentire sensor (and frame store) length. This constitutes a substantialdisadvantage of the CCD. A true window mode, in the sense of a rapidnon-selectable access to the region of interest (ROI), is not thereforepossible. This is the great advantage of a ‘true’ active pixel detectorin which the signal charge is collected and amplified at the point ofgeneration. Here there is no transfer of signal charges, and bycorresponding connection and control of the pixels (image points),arbitrary regions can be selected and read with high repeat rate. Oneexample for the implantation of this concept is a sensor matrixconsisting of DEPFETs (Depleted Field Effect Transistor). Depending onthe size of the ROI, the read speed can be multiplied locally relativeto the entire matrix. The use of active pixels sensors however hasdisadvantages. These include, in particular in DEPFETs, erroneous signaldetection due to the permanent sensitivity (see below) and the so-called“rolling shutter” effects.

These are provoked by the temporal offset on reading of different lines(and hence their integration time) and—in particular with rapidly movingobjects—can lead to artefacts and image distortion.

Since the amplifying electronics integrated in the pixel lie within thesensor region, signal charges received during processing have adifferent effect than in the CCD. As described, in the CCD such eventsare incorrectly assigned in place and time. In the DEPFET, the chargecarriers received at an arbitrary time during the read cycle areincorrectly weighted and thus falsify the detected signal amplitude. TheDEPFET determines the signal by forming the difference between currentsin the transistor of the active pixel before (signal current) and aftera deletion pulse (reference current).

If the signal charges reach the internal gate of the DEPFET afterdeletion, the value of the initial current is falsified, which in theextreme case can even lead to paradoxical “negative” signal amplitudes.

Even more problematic for use however are charges which reach theinternal gate of the DEPFET before deletion, since these areincompletely amplified. Such events are highly problematical inparticular for spectroscopic applications, since the falsified signalamplitudes appear as an irreducible background in the spectrum.

Such signal falsifications are called “misfits” in the jargon. By theirnature, they occur above all in temporally uncorrelated radiation, e.g.on astronomic observations or optical imaging, since here the reading ofthe sensor cannot be synchronized with the incidence of the radiation.The proportion of misfits to total events here corresponds to the ratiobetween the signaling processing time and the integration time.Applications in which the signal rate is high are thereforedisproportionately affected by the problem, since firstly the totalnumber of “misfit” events rises with the signal rate, and secondly thehigh signal rate requires an increase in the image rate, i.e. ashortening of the integration time, with otherwise unchanged signalprocessing time.

Accelerating the read-out by parallelization, e.g. by reading severallines of a matrix simultaneously, also aggravates the problem since theproportion of misfits increases further in proportion to the number ofpixels read simultaneously. The extreme case, sensible from theviewpoint of an experimenter, of a hybrid DEPFET pixel sensor read fullyin parallel is therefore the least favorable from the aspect of spectralusability of data.

This problem is caused by the permanent sensitivity of the pixel evenduring the read-out phase. One solution therefore is to switch the pixelinsensitive during reading. The concept of the DEPFET allows integrationof a conventional electronic shutter. EP 1 873 834 B1 for exampleproposes a DEPFET structure in which the detector can be switchedinsensitive during a definable time window, in that the incidentelectrons are extracted by the deletion contact. An additional electrodesurrounding the internal gate of the DEPFET here prevents extraction ofthe electrons already stored there. This electronic shutter not onlyopens the possibility of controlling the exposure of the sensor withprecision in the microsecond range. In addition, the DEPFET pixel canalso be switched during reading to be insensitive to interferencesignals such as scatter light, thermally generated electrons or evensignal electrons which would lead to misfits.

However this option is associated with a dead time of the total sensorwhich corresponds to the total read-out time. All signal electronsreceived while the shutter is closed are irrevocably lost. Line by lineswitching of the shutter indeed reduces the dead time to the read timeof a line, but the property of the global shutter is lost as a result.

Some applications, e.g. polarimetry, impose further requirements for theelectronic shutter, as well as a purely screening effect. One frequentlyused technique of polarimetry is based on detection of the so-calledStokes parameter. Here separate images are recorded while polarizationfilters are in different positions (typically 4). Since the polarizationsignal consists of the difference between images from two filterpositions, the unpolarized proportion of the light disappears insofar asit does not change while the two images are being recorded. Oftenhowever the light is unavoidably disrupted on the path between sourceand detector. In astronomical observations, it is falsified byturbulence in the upper atmosphere layer. For polarimetry for example,in particular fluctuations in the unpolarized part with frequenciesabout or above the read-out rate are problematical since the unpolarizedpart is greater by many orders of magnitude than the polarized part. Tosuppress the effect of such fluctuations, the polarization plane shouldbe changed and the respective associated image recorded at timeintervals as short as possible.

For these studies, usually CCD-based sensors are used. Some instrumentsuse frame-store CCDs which are operated with as high an image rate aspossible. For each setting of the polarizer, a separate image isrecorded by the CCD. The disadvantage of frame-store CCDs is that thepolarization cannot be changed more quickly than the read-out rate ofthe CCDs. Also 00T-induced errors during shifting into the frame storemust be corrected iteratively. Alternatively, special CCDs are used inwhich lines (typically 4) are covered strip by strip, wherein the linescollect the images of the individual polarizer settings (usually 4).Each polarizer setting is here assigned to a line and the signals ofseveral cycles are cumulated in the respective line before the frame isread. The disadvantage of CCDs with covered columns is that asignificant part of the quantum efficiency is lost, and during read-outthe same errors occur as for frame-store CCDs as long as the CCD is notshaded otherwise (e.g. mechanically). Also both methods are associatedwith significant dead time and the maximum image rate remains coupled tothe read-out speed.

The disadvantages of known detector arrangements therefore comprise thedead times in which the incident radiation is not detected, oralternatively the artefacts generated in the signal by the permanentsensitivity.

Object of the Invention

The object of the invention is to create a detector arrangement with anelectronic shutter which minimizes the dead times in image detection.Also part images can be detected, i.e. signal electrons can be collectedin different time periods in individually assigned stores within animage cell. For the case of periodically recurrent observationconditions, a cumulation of the signals detected directly in therespective store is possible.

SUMMARY OF THE INVENTION

This object is achieved by a detector arrangement and a correspondingoperating method according to the invention.

The essence of the invention is that in each of the pixels of asemi-conductor detector, at least two subpixels are present, each ofwhich can be switched alternately sensitive or insensitive, wherein thesignal charge is collected in the sensitive-switched subpixel while acharge collection in the insensitive subpixel is prevented by potentialbarriers. In a first collection state, the first subpixel is sensitivewhile the second subpixel is insensitive. In a second collection statehowever, the second subpixel is sensitive while the first subpixel isinsensitive.

It should be stated here that with regard to the number of subpixels perpixel, the invention is not restricted to two subpixels per pixel. Forexample, each pixel may have four subpixels or a different number ofsubpixels.

The individual pixels therefore in all cases have a dead time in whichno radiation is measured on deletion of the collected signal chargecarriers. Otherwise the pixels are free from dead time since onesubpixel is always sensitive.

Preferably each subpixel comprises a DEPFET as a read element, but theinvention is not restricted to DEPFETs with regard to read elements.With a DEPFET as a read element in the subpixels, in the insensitivestate potential barriers prevent radiation-generated signal chargecarriers from reaching the internal gate of the insensitive-switchedsubpixel. The potential barriers therefore ensure thatradiation-generated signal charge carriers almost exclusively, with ahigh selectivity, reach the internal gate of the DEPFET in thesensitive-switched subpixel, while the internal gate of the DEPFET inthe insensitive-switched subpixel is shielded by the potential barriers.

The DEPFETs of the individual subpixels can preferably be switchedbetween a read mode and a non-read mode, wherein the signal chargecarriers collected in the internal gate in read mode generate an outputsignal which indicates the measured radiation, whereas in non-read modeno signal is read.

The potential barriers for shielding the insensitive-switched subpixelstart from shielding electrodes within the area of the insensitivesubpixel and extend in the direction of the source region of theinsensitive DEPFET, so that the internal gate of the insensitive DEPFETis shielded from signal electrons. At the same time, the potentialbarriers also prevent the loss of charge already collected from theinternal gate of the insensitive DEPFET.

In a variant of the invention, the DEPFETs of the subpixels within apixel have a common source, wherein the drains of the DEPFETs can thenform the shielding electrode.

In another variant of the invention, the DEPFETs of the subpixels incontrast have a common drain within a pixel, wherein the sources of theDEPFETs can then form the shielding electrodes.

A further variant of the invention however provides separate shieldingelectrodes which are separated from the drains and sources of theDEPFETs and can be controlled separately.

Furthermore, in the context of the invention it is possible for theshielding electrodes of several pixels to be connected togetherelectrically.

It has already been mentioned above that the radiation-generated signalcharge carriers, with high selectivity, only reach the internal gate ofthe DEPFET of the sensitive-switched subpixel, while the internal gateof the DEPFET in the insensitive-switched subpixel is shielded bypotential barriers from the radiation-generated signal charge carriers.The potential barriers for shielding of the internal gate of the DEPFETin the insensitive-switched subpixel are produced firstly by shieldingelectrodes, as has already been briefly explained above. Secondly, thesepotential barriers can also be produced in that the external gates ofthe DEPFETs in the sensitive-switched subpixel on one side and in theinsensitive-switched subpixel on the other are controlled differently.

The selectivity in the charge collection already mentioned is defined asthe ratio between the charge collected by the sensitive subpixel to thecharge collected by the insensitive subpixel of a pixel. A selectivityof 1000 therefore means that for 1001 radiation-generated signal chargecarriers, 1000 of the radiation-generated signal charge carriers arecollected by the sensitive-switched subpixel while only one singleradiation-generated signal charge carrier is collected in theinsensitive-switched subpixel. The invention advantageously allows ahigh selectivity of more than 1000.

Simulations have shown that a high selectivity can be achieved in chargecollection if the surface area proportion of shielding electrodes to thetotal pixel surface area is relatively high. Preferably the surface areaproportion of the shielding electrodes to the total pixel surface areais therefore at least 10%, 20% or even at least 30%.

Furthermore, simulations have shown that a high selectivity can beachieved in the charge collection if the distance between the internalgates (for a common source therefore the lateral extension of the commonsource region), and also the gate lengths of the DEPFETs, are short.With the detector arrangement according to the invention, the lateraldistance between the internal gates of the DEPFETs in the subpixels of apixel is therefore preferably substantially smaller than the lateralextension of the shielding electrodes.

Furthermore, a high potential difference between the two shieldingelectrodes naturally also causes a higher selectivity.

One advantageous embodiment of the invention lies in that the signal ofthe insensitive-switched DEPFET can be read without the reading beingfalsified by charges received during this period and without asubstantial deterioration in the charge collection of thesensitive-switched pixel.

As has already been mentioned briefly above, with regard to the numberof subpixels per pixel, the invention is not restricted to two subpixelsper pixel. For example the invention may also comprise a variant withfour subpixels per pixel, wherein the subpixels are switched sensitivesequentially and are otherwise insensitive. Here each subpixelpreferably has its own DEPFET as a read element. The subpixels may herebe arranged on a common substrate and be separated from each other by aseparator (e.g. of polysilicon).

In addition, the detector arrangement according to the inventionpreferably also comprises a control unit, which in a suitable mannercauses the semi-conductor detector to switch the subpixels to besensitive or insensitive, and to switch between read mode and non-readmode. Firstly, for this the control unit is connected to the shieldingelectrodes in order to produce the potential barriers for shielding theinsensitive pixel. Secondly, the control unit is however also connectedto source or drain and to the external gate of the DEPFETs of theindividual subpixels.

For switching between the different collection states, it is noted thatall DEPFETs of the detector arrangement in the first collection statecan be switched simultaneously to the second collection state. Inaddition all DEPFETs of the detector arrangement in the secondcollection state can simultaneously be switched to the first collectionstate.

Furthermore the individual pixels each have a drift structure whichallows the radiation-generated signal charge carriers to drift to theDEPFETs.

Furthermore it is stated that in each pixel, preferably one subpixel(e.g. with a DEPFET as a read element) is always sensitive, so that theindividual pixels in all cases have a dead time on deletion of thecollected signal charge carrier and are otherwise free from dead time.

In a preferred exemplary embodiment of the invention, the pixels in thedetector arrangement are arranged as a matrix in lines and columns.Switching between the different collection states can here take placeglobally, wherein all pixels of the matrix are switched between thecollection states together, or line by line, wherein all pixels of aline are switched between collection states together. Alternatively itis possible for switching between the different collection states totake place by columns, wherein all pixels of a column can be switchedbetween the collection states together. In addition, switching betweenread mode and non-read mode can take place by lines or columns.

Furthermore, the invention also comprises a corresponding operatingmethod for a semi-conductor detector with several pixels, and in eachcase at least two subpixels per pixel.

As part of the operating method according to the invention, thesubpixels are switched sensitive or insensitive successively, wherein ineach case radiation-generated signal charge carriers are collected inthe sensitive subpixel while the insensitive-switched subpixel isshielded and collects no radiation-generated signal charge carriers.

The signal charge carriers collected in the subpixels are then read in aread mode.

In a variant of the invention, the subpixels are each read in theinsensitive state. The first subpixel is therefore then read in thesecond collection state in which the first subpixel is insensitive whilethe second subpixel is sensitive. The second subpixel is here read inthe first collection state in which the first subpixel is sensitivewhile the second subpixel is insensitive.

In another variant of the invention, the subpixels are however each readin the sensitive state. The first subpixel is therefore then read in thefirst collection state in which the first subpixel is sensitive whilethe second subpixel is insensitive. The second subpixel is here read inthe second collection state in which the first subpixel is insensitivewhile the second subpixel is sensitive.

In the variant described above of a semi-conductor detector with foursubpixels per pixel, the subpixels are preferably switched sensitive andinsensitive in succession. Preferably only one subpixel per pixel issensitive in each case, while the other subpixels of the pixel areinsensitive.

Furthermore it is stated that the sensitive state of the individualsubpixels is preferably set only very short, for example with a durationof less than 1 ms, 100 μs or even less than 10 μs.

Further advantageous refinements of the invention are described in thesubclaims or explained in more detail below with reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an equivalent circuit diagram of a pixel of a detectorarrangement according to the invention, wherein the pixel has twosubpixels.

FIG. 1B shows a cross section view through the pixel according to FIG.1A along section line A-A in FIG. 1D.

FIG. 1C shows the course of electrical field lines and equipotentiallines in the pixel of the detector arrangement according to FIGS. 1A and1B.

FIG. 1D shows a top view of the pixel according to FIGS. 1A-1C.

FIG. 2A shows an adaptation of FIG. 1A.

FIG. 2B shows a corresponding adaptation of FIG. 1B.

FIG. 3 shows a top view of a detector arrangement according to theinvention with two pixels, each having two subpixels, with a driftstructure.

FIG. 4 shows an adaptation of FIG. 1D with additional shieldingelectrodes separate from the DEPFETs.

FIG. 5 shows an adaptation of a pixel with four subpixels.

FIG. 6 shows the connection of several pixels, each with two subpixels,in a sensor matrix.

FIGS. 7A and 7B show the sequence of the different collection states forthe sensor matrix in FIG. 6.

FIG. 8 shows the connection of several pixels, each with four subpixels,in a sensor matrix.

FIGS. 9A-9D show the sequence of the different collection states in thesensor matrix according to FIG. 8.

DETAILED DESCRIPTION OF THE DRAWINGS

In the simplest case, according to FIGS. 1A-1D, a pixel of the detectorstructure according to the invention consists of two adjacent DEPFETs 1,2 each with a common source S, each with a drain D1, D2, each with aninternal gate IG1, IG2, and each with an external gate G1, G2, whereinthe DEPFETs 1, 2 are located on the surface of a weakly n-doped(high-impedance) silicon wafer 3 and each form a subpixel. FIGS. 1B and1D show a cross section and the associated layout (top view).

The silicon wafer 1, in the region of a sensor matrix consisting of theDEPFETs 1, 2, has on the back a heavily p-doped thin layer 4. The actualDEPFETs 1, 2 are arranged on the top of the silicon wafer. They areformed by external (MOS) gates G1, G1 with dielectric 5, 6. The sourceregion S common to the two DEPFETs 1, 2 and the drains D1, D2 delimitthe transistor channels, below which are the internal gates IG1 and IG2.These serve as collection electrodes on collection of the signal chargegenerated in bulk, and as additional electrodes controlling the channelson read out.

The two DEPFETs 1, 2 together with a deletion structure L (see FIG. 1D)form a pixel with two subpixels. The charge incident in this region isassigned to this pixel. The two DEPFETs 1, 2, in the internal gates IG1,IG2 of which the charge is stored alternately, are designated subpixelsin the context of the invention. A sensor usually consists of amatrix-like arrangement of pixels.

The operation of the pixel according to the invention is describedbelow. In this exemplary embodiment, the focus lies on minimizing deadtime during reading of the DEPFET matrices, and the function of theshielding electrodes is assumed by the drains D1, D2 of DEPFETs 1, 2. Aconnection of the pixels within a matrix according to the exemplaryembodiment described below is shown in FIG. 6.

Here control lines SLD1 and SLD2 serve to control the drains D1 _(ij)and D2 _(ij) of the individual subpixels. Each drain D1 _(ij) and D2_(ij) of the individual subpixels is here connected to one of the twocontrol lines SLD1 and SLD2, in the individual columns alternately.

Furthermore each line of the matrix has two control lines SLG1 _(i) andSLG2 _(i) which are connected to gates G1 _(ij) and G2 _(ij) of allpixels of the respective line.

Finally the common sources S_(ij) of the pixel are each connected withina column to an output line O1, O2 or O3 per column.

Charge Collection and Charge Storage

During all operating phases, the silicon wafer 1 is depleted. Thedepletion (charge carrier depletion) results from a relatively highnegative voltage V_(B) applied to the rear layer 4 in interaction withthe depletion effect of the p⁺-doped drains D1, D2 and the also p⁺-dopedsource S and the deletion process.

On collection of the signal charge, the two subpixels are switched offi.e. the external gates G1, G2 are set to the relatively positivepotential V_(G1) or V_(G2) compared with a reference potential V_(S) ofsource S. For the sake of simplicity, the threshold voltages of theDEPFETs 1, 2 with empty internal gates IG1, IG2 lie at 0V, so that theDEPFETs 1, 2 are blocking. It is characteristic of the invention thatthe drains D1, D2 of the two subpixels which also serve as shieldingelectrodes lie at different potentials. The one drain D1, as normal withp-channel transistors, lies at a negative potential (V_(D1)=−5V). Theother drain D2 is switched to a potential which is positive relative tothis (V_(D2)=0V).

FIG. 1C shows the resulting, simulated two-dimensional potentialdevelopment within a pixel structured according to FIGS. 1B and 1D, insection A-A. Since the simulations relate to a right-hand coordinatesystem, the surface of the silicon wafer 3 with DEPFETs 1, 2 lies atcoordinate y=0, and the back with the p⁺-doped layer 4 lies at y=450 μm.The potential distribution within the first 100 μm is shown. Thecontinuous lines are lines of equal potential (isopotential lines). Thedotted electric field lines run perpendicular to these. They form thetrajectories for the signal electrodes and were selected so that theysweep over the semi-conductor below the pixel. They all end in theinternal gate IG2 of the right subpixel, the drain D2 of which lies at0V, even when the trajectories start below the left subpixel. Theinternal gate IG1 of the left subpixel does not collect any charge.

As already stated, in this exemplary embodiment the drains D1, D2function as shielding electrodes. They repel the charge from the leftsubpixel and conduct it to the right subpixel. It is an essential partof the invention that the roles of the two pixels are exchanged. Forthis it is only necessary to switch the voltage values present at theshielding electrodes. Subpixels of which the shielding electrodes lie ata positive potential and which can therefore collect electrons, arereferred to below as sensitive. In contrast, subpixels with shieldingelectrodes lying at negative potential i.e. with shielded internalgates, are insensitive. The advantage of this sensor component willbecome clear in consideration of the read process.

Reading

As described in the prior art, DEPFET matrices are preferably read lineby line in rolling shutter mode. The image stored in the entire matrixis called a frame. In a matrix with n lines, consequently n read outprocesses are required to obtain a frame. To read a line, thecorresponding gate is activated so that the DEPFETs of this line becomeconductive.

For a current read-out, the signal current, i.e. the transistor currentmodulated by the charge in the internal gate IG1 or IG2, is detected andtemporarily stored by read electronics arranged at the edge of thematrix.

In the alternative voltage read-out (source follower), the active DEPFET1 or 2 transfers the read line which connects the transistor to the readelectronics. The signal voltage value is then detected and temporarilystored.

Then the reference value is determined. For this the charge is removed(deleted) from the internal gate IG1 or IG2 using the deletionstructures L (see FIG. 1D). The current or voltage value then measuredrepresents the charge state of the empty internal gate IG1 or IG2. As ameasure of the charge previously collected, in the read electronics thedifference is formed from the signal and reference value i.e. the valuesbefore and after deletion. This known technique is called “correlateddouble sampling” (CDS).

As already explained, signals which reach the internal gate IG1 or IG2during reading are not detected correctly because either the signal orthe reference value is falsified by the incident charge carriers, whichresults in erroneous values in the difference formation.

In the pixel according to the invention (in this exemplary embodiment)there is a sensitive and an insensitive subpixel. In the insensitive(left) subpixel, a negative voltage lies at the drain D1, whereby theinternal gate IG1 is shielded. At the same time however, the subpixelcan be read by activating the associated gate G1. This ensures thatduring the reading process, no charge can reach the internal gate IG1and so-called “misfit” events are efficiently suppressed. In contrast toa deletion which is based solely on an electronic shutter, the signalcharge received during the read time is not lost but is conducted to theinternal gate IG2 of the other subpixel, now sensitive.

In this exemplary embodiment, the voltages are switched globally at theshielding electrodes, in this case drains D1, D2. At all points on thematrix, simultaneously insensitive subpixels become sensitive subpixelsand vice versa. The matrix reading (frame reading) can then take placeline by line as usual. For this, in each pixel of an activated matrixline, only the insensitive subpixel is read while the sensitive subpixelcan continue to collect signals. If necessary, an additional integrationtime follows the reading process, during which signals are onlycollected but are not read. In this state all external gates G1 and G2are switched off.

When reading is complete, before the start of the next read cycle, theroles of the subpixels are exchanged again by global switching of theshielding electrodes (i.e. drains D1, D2). The previously sensitivesubpixels now store both the charge collected during the lastintegration time and the signal charge which was received during thepreceding read cycle. The latter image information is also detectedunfalsified in the pending read cycle.

In a sensor based on the detector structure according to the inventionand operated as described above, no misfits can occur and theirsuppression is not associated with additional dead time. Real advantagesare therefore offered, in particular on repeated detection of ROIs withhigh image rate and in reading of sensors with a high degree ofparallelization.

The former can indeed be implemented with conventional DEPFETs but,because of the unfavorable ratio of integration to read time, thespectroscopic quality is significantly poorer than for the full frameread-out.

The same also applies for reading sensors with high degree ofparallelization. In a “conventional” DEPFET, the proportion of misfitsis directly proportional to the number of pixels read simultaneously.The least favorable case therefore corresponds to the fully parallelread-out, i.e. a hybrid pixel sensor in which all pixels are readsimultaneously. A sensor based on the detector structure according tothe invention however supplies usable data with simultaneously goodspectroscopic quality even for a maximum degree of parallelization.

In addition, the design described of such a sensor according to theinvention has a further advantage. Because of the simultaneous globalswitching, image detection is instantaneous, whereby no artefacts canoccur due to “rolling shutter” effects. These have a disruptive effectin the optical sense in particular on imaging of rapidly changingobjects.

The switching of the shielding electrodes (i.e. drains D1, D2) can takeplace very quickly. Indeed all control lines leading to the shieldingelectrodes must be switched simultaneously, which leads to a highcapacitive load, but for this special driver chips can be used, of whichonly two are needed per matrix. This situation is comparable with theglobal switching of registers in a CCD, for which normally significantlyless than 100 ns are necessary. However the switching of the subpixelsneed only take place once per read cycle, whereas for a three-phase CCD,three cycles per line read are required. Thus the loss power balance issubstantially more favorable for the DEPFET matrix.

Naturally it is possible that the signal charge which reaches the pixelduring the short switching process is split over two subpixels. Thischarge is not however lost. If necessary it can be correctlyreconstructed on subsequent analysis of the data.

The two subpixels are reset via common deletion structures L whichconnect to the internal gates IG1, IG2 on the left and right (see FIG.1D). This is a very compact layout, in particular because the number ofcontrol lines and the number of driver chips for the deletion processare halved. However it is assumed that the insensitive subpixels canretain their charge during the deletion process, which is guaranteed bya sufficiently positive potential of the internal gates of thesesubpixels. For technological reasons however it is possible that thispositive potential cannot be set, e.g. because the associated electricalfields trigger charge carrier multiplications in the region of theinternal gate. Then the more complex arrangement should be selected, inwhich each subpixel has its independently controllable deletionstructure.

FIGS. 2A and 2B show an adaptation of the exemplary embodiment fromFIGS. 1A-1D, so to avoid repetition, reference is made to thedescription above, wherein for the corresponding details the samereference numerals are used.

One feature of this exemplary embodiment is that the two DEPFETs 1, 2have a common drain D and separate sources S1, S2.

There is therefore a further possibility of improving the selectivity ofthe charge collection between the sensitive and insensitive subpixels byswapping the roles of drain and source. For this, the two subpixels ofthe common p⁺-doped region are not—as in the former exemplaryembodiments presented—laid to source voltage but to the negative drainvoltage. On reading, the signal is again taken from here. The externalsources S1, S2 admittedly continue to function as shielding electrodesand are again switched between a positive (sensitive subpixel) and anegative voltage (insensitive subpixel). The shielding electrodes of theinsensitive subpixel can here be switched significantly more negativelythan the common drain D. Since in this way the potential differencebetween the shielding electrodes (i.e. the sources S1, S2) may begreater than the source-drain voltage of the DEPFETs 1, 2, significantlyhigher values can be achieved for selectivity.

For reading, the shielding electrodes S1, S2 must however assume thefunction of the source, and for this they must be laid to a positivevoltage relative to drain D, e.g. 0V. This corresponds however to thesensitive state i.e. the pixel can only be read in sensitive state.

Thus the advantage of misfit suppression is lost. The advantage of thestructure lies in the significantly increased selectivity of chargecollection. Furthermore, for this structure, a current-based read-out ofsignals may be implemented at drain D, which offers advantages relativeto the achievable read-out speed with significantly lower cost.

The use of such a detector structure is advantageous above all when thesensor can be read after end of the integration phase, or if a highselectivity is required, while the influence of misfits in the spectrumis negligible.

The exemplary embodiment according to FIG. 3 also partially correlateswith the exemplary embodiments described above, so to avoid repetition,reference is made to the description above, wherein for correspondingdetails the same reference numerals are used.

Thus FIG. 3 shows the exemplary embedding of two of the pixels describedabove, each with two subpixels, in a drift structure DS. Evidently otherforms of drift structure and also several concentrically arranged,annular drift structures can be used.

Very usefully, drift structures DS can also be implemented by steppedimplantations, wherein implantation steps are applied which are used inany case in technological production processes. The spatial chargesconnected with the implantations can focus the signal charge in thedirection of the internal gates IG1 or IG2, or may be used to delimitthe pixels.

Also, the location and connection of the pixels within the matrix affectthe selectivity. With the design shown in FIGS. 1A-1D, amirror-symmetrical connection relative to the left and right (FIG. 1B),or to the upper and lower pixel limit (FIG. 1D), leads to pixels whichare adjacent in the read direction splitting the drain regions D1, D2functioning as shielding electrodes. This gives a high selectivitybecause the surface areas of this shielding electrodes are de factodoubled. On reading, it must be taken into account that the pixelsborder each other alternately with their sensitive and insensitivesubpixels. The sequence of control of the different subpixels and theirassociation within the matrix is shown in FIGS. 7A and 7B.

As shown in FIG. 1D, the shielding electrodes are not interruptedtowards their lateral neighbors, which leads to an additional increasein surface area. The signal is read at the source S.

The connection of the pixels within a sensor matrix is shown in FIG. 6.

A further, very effective possibility of increasing selectivity lies inlowering the potential of the internal gate IG1 of the insensitivesubpixel during collection relative to that of the sensitive internalgate IG2. This can be achieved very easily via the external gate G1. Indisconnected state, there is no deletion channel in DEPFET 1 so theexternal gate G1 can capacitively access the internal gate IG1. Thiscapacitive coupling allows the potential of the internal gate IG1 to beset via the voltage at the external gate G1. For example, the followingvoltages can be set at the external gate G1, G2:

-   -   insensitive pixel in read state: −3V    -   insensitive pixel in collection state: 1V    -   sensitive pixel in collection state: 3V.

In the simulation example shown above (FIG. 1C), the selectivity isimproved over standard operation (collection state: external gate ofinsensitive subpixel=3V) by almost an order of magnitude from 9.4e04 to8.5e05. This improvement is naturally achieved at the cost of a morecomplex control system. One possible technical solution is to provide a3-level cycle rate for the control chip for the gate lines.

The read time of a frame, in operation of the matrix in rolling shuttermode, is equal to the sum of the read times required for the individuallines. This limits the time resolution of a detector, which cannot becompensated in all application cases even by the parallel reading ofseveral lines or by the introduction of ROIs. In these cases, the use ofpixels according to the invention, due to the suppression of misfits,indeed leads to a significant improvement in the spectroscopic qualityof the signals but the problem of the low time resolution is noteliminated.

In the hybrid DEPFET pixel sensor, using 3D integration techniques aseparate read amplifier is assigned to each DEPFET pixel. Theseamplifiers are also arranged as a matrix within a read chip. The readprocess may therefore be carried out completely in parallel with maximumpossible time resolution. As already stated, this is however the leastfavorable case from the viewpoint of “misfit suppression”. The use ofthe pixel according to the invention as a sensor element is particularlyadvantageous here. It is suitable to connect the source region common tothe two subpixels to the input of the assigned amplifier via a bumpbond.

The exemplary embodiment in FIG. 4 also partly correlates with theexemplary embodiments described above, so to avoid repetition, referenceis made to the description above, wherein for corresponding details thesame reference numerals are used.

This exemplary embodiment has separate shielding electrodes SH1, SH2which are separate from the drains D1, D2. The shielding electrodes SH1,SH2 therefore need not necessarily be formed by the drains D1, D2 of theDEPFETs 1, 2. If the faster current read-out is required in a detector,usually drains D1, D2 are connected to the amplifiers of the electronicread chip. If however sampling takes place over a wider voltage range,precautions must be taken to decouple the amplifier input. This can beavoided if the shielding function is transferred to the separateshielding electrodes SH1, SH2 arranged further out. For this, as shownin FIG. 4, drift structures are suitable. To guarantee an effectivepotential access to the regions below the internal gates IG1, IG2, thedrain regions D1, D2 lying in-between should be relatively narrow. Oneadvantage of this arrangement is that the choice of voltage differencesat the shielding electrodes SH1, SH2 may be independent of thetransistor function. They may lie in the range of 10V or 20V, wherebythe greater distance from the internal gate IG1 or IG2 can becompensated. The more positive shielding electrode voltage for thesensitive-switched subpixel should not however be more positive than thedrain voltage, because otherwise potential barriers for the signalelectrons would be constructed.

The exemplary embodiment in FIG. 5 also partly correlates with theexemplary embodiments described above, so to avoid repetition, referenceis made to the description above, wherein for the corresponding detailsthe same reference numerals are used.

It should be stated here that the switching of the shielding electrodescan take place more frequently than the reading of the imageinformation. Since this takes place very quickly and with no loss ofsignal charge, signal charges can therefore be assigned to m differenttemporal phases of the image detection. Here m is the number ofsubpixels of a pixel. The switching of the shielding electrodes heretakes place simultaneously for the entire matrix. A typical applicationfor imaging polarimetry would use 4 subpixels, as shown in FIG. 5.

In contrast to the arrangement shown in FIGS. 1A-1D, the two subpixelsare separated by a separator 7 which is preferably formed by a firstpolysilicon layer. The external gates G1, G2 of the DEPFETs are formedfrom a second polysilicon layer deposited later. In production, thefirst polysilicon layer of the separator 7 blocks off the implantationswhich form the channels and the internal gates, so that these areseparated and internal gates are formed within a pixel. The separator 7also splits the p⁺-implanted shielding electrodes (drains) and thep⁺-implanted source region, whereby finally four subpixels result, eachwith a shielding electrode.

The separator 7 is laid to a slightly positive voltage of approx. 1V inorder to suppress parasitic channels between the p⁺-doped regions. Onuse of an additional n implantation in the silicon covered by theseparator 7, this voltage shifts to more negative values.

For this in a polarimeter, in synchrony with the switching of themodulator, in each case precisely one of the four shielding electrodesis switched to collection mode (0V) while all others repel the charge(shielding electrodes at −5V). If, for example during the firstpolarizer setting, the signal charge is collected in the first subpixel,simultaneously with switching of the polarizer via the second shieldingelectrode, the second subpixel is switched sensitive and the firstinsensitive via its shielding electrode. The process is the same for thethird and fourth polarizer settings. When the first polarizer settingcomes round again, the signal charge is added to the signal electronsalready present in the first subpixel. This cycle is continued untilsufficient photons have been collected or the dynamic range of theDEPFET is exhausted. If the modulator has four positions, four separateDEPFET stores are required per pixel in order to store the imageintensities of the four modulator settings. With this method, the foursubpixels are filled with signal charges in close temporal correlation,so that on later subtraction of the images, higher frequencyinterference is also deleted.

As in the first exemplary embodiment, here too it is advantageous toimprove the selectivity by combining the shielding electrodes ofadjacent subpixels beyond the pixel boundaries. If the pixel shown inFIG. 5 is connected mirror-symmetrically in both directions relative tothe pixel boundaries, we find that the surface areas of the combinedshielding electrodes are four times larger than the area of a singlesubpixel-related shielding electrode. Connection of the pixels in amatrix arrangement is shown in FIGS. 8 and 9A-9D. The enlarged shieldingelectrodes rotate around the pixel on switching, as illustrated by theimage sequence contained in FIGS. 9A-9D.

Here four control lines SLD1, SLD2, SLD3 and SLD4 are shown, wherein inall pixels of the matrix, the drains of the four subpixels are eachconnected to one of the control lines SLD1, SLD2, SLD3 and SLD4.

Furthermore each line of matrix has two control lines SLG1 i and SLG2 iin order to control the common gates of each two subpixels of the pixelof the respective line.

Finally FIG. 8 shows six output lines 01-06 which are each connected incolumns to the common sources of the subpixels of the respective column.

Each signal read-out can take place for the subpixels which are alreadyin the insensitive state. During polarizer setting one or two forexample, subpixels three and four can be read and vice versa. Reading ofa line normally takes substantially less time than collection, for thespeed with which the polarizer setting can be changed is decisive forthe duration of a collection phase. Usually, the changing of thepolarizer plane limits this integration time. Therefore several linescan be read during a polarizer setting.

After a few cycles therefore the entire matrix is read withoutinterrupting the charge collection of the other DEPFETs. In this way adead time of the system can be avoided.

The invention is not restricted to the preferred exemplary embodimentsdescribed above. Rather a plurality of variants and adaptations arepossible which also make use of the inventive concept and therefore fallwithin the scope of the protection.

LIST OF REFERENCE NUMERALS

-   1 DEPFET-   2 DEPFET-   3 Silicon wafer-   4 Back layer of silicon wafer-   5 Dielectric-   6 Dielectric-   7 Separator-   D Drain-   D1, D1′ Drain-   D2, D2′ Drain-   DS Drift structure-   G1, G1′ External gate-   G2, G2′ External gate-   IG1 Internal gate-   IG2 Internal gate-   L, L′ Deletion structure-   S, S′ Source-   S1 Source-   S2 Source-   SH1 Shielding electrode-   SH2 Shielding electrode-   SLD1 Control line for drains-   SLD2 Control line for drains-   SLD3 Control line for drains-   SLD4 Control line for drains-   SLG1 i Control line for gates-   SLG2 i Control line for gates-   O1-O6 Output lines-   V_(B) Voltage at rear layer 4 for depletion of silicon wafer 3-   V_(S) Potential of source-   V_(G1) Potential of external gate G1-   V_(G2) Potential of external gate G2-   V_(S) Potential of common source S-   V_(S1) Potential of source S1-   V_(S2) Potential of source S2-   V_(D) Potential of common drain D-   V_(D1) Potential of drain D1-   V_(D1) Potential of drain D2

1. A detector arrangement for detection of radiation, comprising: a) asemi-conductor detector with several pixels for detection of theradiation, wherein b) each of the pixels each has a first subpixel and asecond subpixel, and c) the semi-conductor detector is switchablebetween c1) a first collection state in which the first subpixel issensitive and the second subpixel is insensitive, so thatradiation-generated signal charge carriers are collected substantiallyonly in the first subpixel, and c2) a second collection state in whichthe second subpixel is sensitive and the first subpixel is insensitive,so that the radiation-generated signal charge carriers are collectedsubstantially only in the second subpixel.
 2. The detector arrangementaccording to claim 1, wherein a) the pixels have a dead time in allcases on deletion of collected signal charge carriers and are otherwisefree from dead time, and b) for each of the pixels always one of thesubpixels is sensitive so that apart from deletion of collected signalcharge carriers, the pixels are free from dead time.
 3. The detectorarrangement according to claim 1, wherein a) the first subpixel has afirst DEPFET with an internal gate for collection of radiation-generatedsignal charge carriers, and b) the second subpixel has a second DEPFETwith an internal gate for collection of radiation-generated signalcharge carriers, and c) in the first collection state, theradiation-generated signal charge carriers substantially only reach theinternal gate of the first DEPFET, while the internal gate of the secondDEPFET is shielded, so that substantially none of theradiation-generated signal charge carriers reaches the internal gate ofthe second DEPFET, and d) in the second collection state, theradiation-generated signal charge carriers essentially only reach theinternal gate of the second DEPFET, while the internal gate of the firstDEPFET is shielded, so that essentially none of the radiation-generatedsignal charge carriers reaches the internal gate of the first DEPFET. 4.The detector arrangement according to claim 3, wherein the DEPFETs caneach be switched between a) a read mode in which the signal chargecarriers collected in the internal gate of the DEPFET to be read,generate an output signal which indicates the measured radiation, and b)a non-read mode in which no signal is read.
 5. The detector arrangementaccording to claim 3, further comprising shielding electrodes forshielding the internal gate of the DEPFET in the insensitive-switchedsubpixel, so that the radiation-generated signal charge carrierssubstantially do not reach the internal gate of the DEPFET in theinsensitive-switched subpixel.
 6. The detector arrangement according toclaim 5, wherein the DEPFETs of the subpixels have a common source anddrains of the DEPFETs form the shielding electrodes.
 7. The detectorarrangement according to claim 5, wherein the DEPFETs of the subpixelshave a common drain and sources of the DEPFETs form the shieldingelectrodes.
 8. The detector arrangement according to claim 5, whereinthe shielding electrodes are separate from drains and sources of theDEPFETs and can be controlled separately.
 9. The detector arrangementaccording to claim 3, wherein a) different pixels of the semi-conductordetector each have at least one shielding electrode for each of thesubpixels, for shielding of the internal gate of the insensitiveDEPFETs, and b) shielding electrodes of several pixels are connectedtogether electrically.
 10. The detector arrangement according to claim3, wherein a) the external gate of the sensitive DEPFET is controlleddifferently from the external gate of the insensitive DEPFET, and b) thediffering control of the external gates of the DEPFETs in thesemi-conductor detector contributes to a potential field which conductsthe radiation-generated signal charge carriers substantially only intothe internal gate of the sensitive DEPFET and shields the internal gateof the insensitive DEPFET.
 11. The detector arrangement according toclaim 1, wherein a) the radiation-generated signal charge carriers reachthe internal gate of the sensitive-switched DEPFET with a selectivitysuch that they are more than 1000 times more numerous than thosereaching the internal gate of the insensitive-switched DEPFET, and b)the detector arrangement for a pixel takes up a certain total pixelsurface area of a semi-conductor substrate, wherein the shieldingelectrodes take up a surface area proportion of at least 10% of thetotal pixel surface area, in order to achieve a high selectivity oftheir shielding effect, and c) a lateral distance between the internalgates of the DEPFETs is substantially smaller than a lateral extensionof the shielding electrodes, in order to achieve a high selectivity oftheir shielding effect.
 12. The detector arrangement according to claim1, wherein a) each of the pixels has at least four subpixels which aresequentially switched sensitive and are otherwise insensitive, and b)each of the subpixels has a DEPFET, and c) the subpixels are arranged ona common substrate and separated from each other by a separator.
 13. Thedetector arrangement according to claim 3, wherein a) the detectorarrangement has a control unit which is electrically connected to theDEPFETs, and b) the control unit switches between the first collectionstate and the second collection state, and c) the control unit controlsthe shielding electrodes electrically in order to switch between thefirst collection state and the second collection state, and d) thecontrol unit switches the DEPFETs between read mode and non-read mode,and e) the control unit controls the source or drain and external gateof the DEPFETs to switch between read mode and non-read mode.
 14. Thedetector arrangement according to claim 3, wherein a) all DEPFETs of thedetector arrangement which are in the first collection state areswitched simultaneously to the second collection state, and b) allDEPFETs of the detector arrangement which are in the second collectionstate are switched simultaneously to the first collection state, and c)the pixels contain a drift structure which allows theradiation-generated signal charge carriers to drift to the DEPFETs, andd) always at least one of the DEPFETs is sensitive, and e) the DEPFETshave a dead time in all cases on deletion of the collector signalcarriers and are otherwise free from dead time.
 15. The detectorarrangement according to claim 1, wherein a) the pixels are arranged asa matrix in lines and columns, and b) all pixels of the matrix or thepixels of individual lines or individual columns are switched betweenthe first and second collection states together, and c) the pixels ofthe individual lines or individual columns are switched between readmode and non-read mode together.
 16. An operating method for a detectorarrangement with semi-conductor detector with several pixels, each ofwhich has at least two subpixels, comprising the following method steps:a) setting of a first collection state of the semi-conductor detector inwhich the first subpixel is sensitive while the second subpixel isinsensitive, b) collection of radiation-generated signal charge carriersin the sensitive first subpixel, c) setting of a second collection stateof the semi-conductor detector in which the second subpixel is sensitivewhile the first subpixel is insensitive, and d) collection ofradiation-generated signal charge carriers in the sensitive secondsubpixel.
 17. The operating method according to claim 16, furthercomprising the following method steps: a) reading of collectedradiation-generated signal charge carriers from the first subpixel in aread mode, and b) reading of the collected radiation-generated signalcharge carriers from the second subpixel in a read mode.
 18. Theoperating method according to claim 17, wherein the read mode of thefirst subpixel takes place during the second collection state in whichthe first subpixel is insensitive, while the read mode of the secondsubpixel takes place during the first collection state in which thesecond subpixel is insensitive.
 19. The operating method according toclaim 17, wherein the read mode of the first subpixel takes place duringthe first collection state in which the first subpixel is sensitive,while the read mode of the second subpixel takes place during the secondcollection state in which the second subpixel is sensitive.
 20. Theoperating method according to claim 16, wherein a) at least four DEPFETsare switched sequentially sensitive or insensitive, and b) in each caseone of the DEPFETs is switched sensitive while another DEPFET isswitched insensitive.
 21. The operating method according to claim 20,further comprising the following steps: a) setting of the firstcollection state of the semi-conductor detector in which only the firstsubpixel is sensitive while the other subpixels are insensitive, b)collection of radiation-generated signal charge carriers in thesensitive first subpixel, c) setting of a second collection state of thesemi-conductor detector in which only the second subpixel is sensitivewhile the other subpixels are insensitive, d) collection ofradiation-generated signal charge carriers in the sensitive secondsubpixel, e) setting of a third collection state of the semi-conductordetector in which only a third subpixel is sensitive while the othersubpixels are insensitive, f) collection of radiation-generated signalcharge carriers in the sensitive third subpixel, g) setting of a fourthcollection state of the semi-conductor detector in which only a fourthsubpixel is sensitive while the other subpixels are insensitive, and h)collection of radiation-generated signal charge carriers in thesensitive fourth subpixel.
 22. The operating method according to claim21, wherein the read mode of the individual subpixels takes place in theinsensitive state.
 23. The operating method according to claim 21,wherein the read mode of the individual subpixels takes place in thesensitive state.
 24. The operating method according to claim 16, whereina) in the individual DEPFETs, the sensitive state is set for a durationof less than 1 ms, 100 μs or 10 μs, and b) switching between theindividual measurement states takes place more frequently than reading.